The present invention concerns a procedure which is applicable in determining the composition of wood chip mixes dispensed in alkaline wood delignification, particularly in sulphate digestion, by observing the changes in concentration of the decomposition products formed from the wood material by cleaving and dissolved in the cooking liquor.
Wood chip mix is in this connection understood to mean, in the first place, chip mixes consisting of softwood and hardwood. The procedure is applicable both in so-called batch cooking processes and in continuous cooking processes. In the latter case it also becomes possible to observe the shift of the wood species boundary taking place after a change of wood raw material: this affords highly valuable, and frequently indispensable, information needed in order to maintain uniform quality. Since the delignification rates of softwood and hardwood are different, the information gained concerning chip composition contributes substantially to success in endeavours to optimate the process.
In alkaline digestion processes lignin contained in the wood raw material, which binds the cellulose fibres together, is removed under strongly alkaline conditions, whereas also partial decomposition of the polymeric carbohydrate material of the wood (cellulose and hemicelluloses) to aliphatic carboxylic acids takes place at the same time (Sjostrom, E., Wood Chemistry; Fundamentals and Applications, Academic Press, New York, 1981). It is thus understood that the organic matter dissolved in the waste liquor is composed not only of lignin decomposition products but also of said carbohydrate decomposition products, and in minor quantity, of wood extractives. The greater part of said decomposition products are present in the form of monomeric compounds which can be analytically separated by means of chromatographic methods.
As taught by earlier patent applications (FI 850208 and FI 870312), corresponding to U.S. Pat. Nos. 4,853,084 and 4,944,841 respectively control of alkaline digestion processes can be implemented, with surprising exactitude, by chromatographically analyzing the relative composition of aliphatic carboxylic acids or lignin monomers contained in the cooking liquor.